1. Field of the Invention
The invention relates to color management systems, and more particularly relates to measurement-based color management systems that construct measurement-based color transforms from transform-based profiles.
2. Description of the Related Art
Color management systems (CMS) convert color between color device representations of a source and a destination color device. To perform this conversion, color values for a color space of the source device are converted into color values for a device-independent color space, and thereafter, these device-independent color values are converted into values for a color space of the destination device.
Some CMSs utilize transform-based color profiles for the conversion of color image data between device-dependent and device-independent color spaces. Such transform-based color profiles are often created by a hardware manufacturer of a given color device, such as a printer or a digital camera. Typically, transform-based color profiles are designed to meet the specifications set forth by the International Color Consortium (ICC).
A color transform contained in a transform-based profile can be used by a color management module (CMM) of the CMS to convert device-dependent color values into color values for a device-independent color space. For example, device-dependent RGB values used by a source device (e.g., a digital camera) can be converted, using the color transform, to device-independent color space values (e.g., LAB values). This device-independent color space is sometimes referred to as the Profile Connection Space (PCS). The PCS values can then be converted to a device-dependent color space of a destination device (e.g., a printer) by utilizing the same CMM in combination with the destination device's transform-based color profile.
Transform-based color profiles often contain multiple color transforms. Transforms are typically represented by combinations of matrices, 1-dimensional look-up tables (LUT), and n-dimensional LUTs for converting color values to PCS values (source transform), and for converting PCS values to a device-dependent color space, e.g., RGB, CMYK, (destination transform). The purpose of multiple transforms is to emphasize different rendering intents. Examples of different rendering intents are perceptual, saturation, relative-colorimetric, and absolute-colorimetric.
Because a source device (e.g., a digital camera) and a destination device (e.g., a printer) typically have different color gamuts with respect to each other, a gamut mismatch may occur, meaning that some colors within the source device's color space may not be represented within the destination device's color space. To achieve an intended reproduction of source colors on the destination device, color transforms typically encapsulate rendering “intents” (or rendering “models”) used to render the image by mapping out-of-gamut colors from within the source device's color gamut to within the destination device's color gamut. For example, colorimetric transforms encapsulate colorimetric rendering intent which provides accurate color reproduction, and perceptual transforms encapsulate perceptual rendering intent which maps colors to preserve a preferred look.
For colorimetric intent, the PCS is an unbounded color space. Colorimetric transforms encapsulate colorimetric intent rendering models that attempt to maintain a near exact relationship between PCS colors and destination device colors. While colorimetric destination transforms change colors, colorimetric source transforms generally do not. The destination colorimetric transform rendering model performs rendering by converting colors that fall outside the destination device's color gamut to colors that can be represented by the destination device. All other colors are converted without change.
For perceptual intent, the PCS is a bounded color space that is limited to color values that can be reproduced on an idealized reference medium (i.e., the PCS perceptual intent reference medium). The PCS perceptual intent reference medium (PRM) and its color gamut (PRMG) have been standardized by the International Color Consortium (ICC).
Perceptual intent transforms encapsulate perceptual intent rendering models that map out-of-gamut colors to colors within the destination device's color space as part of a two-stage rendering process. In the first stage, source colors are rendered from the source device to the PRM, followed by the subsequent rendering from the PRM to the destination device. Rendering in the first stage is achieved using the source transform included in the color profile of the source device, and rendering in the second stage is achieved using the destination transform included in the color profile of the destination device. In both of these renderings, the color mapping is done in such a manner as to obtain a pleasing or otherwise desirable result.
Two gamut maps are used to map colors during creation of the source and destination profiles. When the source device profile is created, a profile builder applies a first gamut map to the source transform to map source device colors to colors within the PRMG. When the destination device profile is created, the profile builder applies a second gamut map to the destination transform to map colors within the PRMG to destination device colors. The source profile is created without knowledge of a destination device, and the destination profile is created without knowledge of a source device.